1. Field of the Invention
The present invention relates generally to integrated circuits, and more particularly to a structure and method for fabricating integrated circuits which provides for the detection of residual conductive material.
2. Description of the Prior Art
Complex integrated circuits typically contain multiple components on one device. Consequently, multiple layers of interconnect are needed to connect various sections of the integrated circuit together. A problem arises, however, when fabricating a polycrystalline silicon interconnect layer over an underlying interconnect layer or an insulating layer having steps or sidewalls. When the polycrystalline silicon interconnect layer is patterned, residual conductive material of polycrystalline silicon is formed adjacent to the sidewalls of the underlying interconnect or insulating layer. Residual conductive material can also be left in small geometric areas on the integrated circuit. This residual conductive material can connect sections of the overlying polycrystalline silicon interconnect layer together. And, because the residual conductive material is typically undoped or lightly doped, it has an intrinsic or near intrinsic resistivity. These resulting features can cause various reliability problems with the integrated circuit.
One method used to try and remove the residual conductive material is overetching the integrated circuit during fabrication. Those skilled in the art will recognize that significant overetching is required to ensure removal of the residual conductive material. Significant overetching, however, has the undesirable effect of damaging other materials. Consequently, the length of an etch is selected by balancing between the need to remove the residual conductive material, and the need to minimize the collateral damage. As a result of this balancing, residual conductive material will occasionally remain in an integrated circuit.
As stated above, the residual conductive material can cause reliability problems with an integrated circuit. If the integrated circuit is an SRAM, for example, residual conductive material can cause single bit failures in the individual cells. Alternatively, it can cause the SRAM cell to operate in a narrow voltage range or operate unreliably. Those skilled in the art will recognize that the resistivity of the residual conductive material can change over time. This change in resisitivity can cause the voltage range at which the SRAM operates to fluctuate, resulting in a flaky bit.
A SRAM cell which is known to have a problem may be salvaged by utilizing redundancy or laser repair. However, situations may still exist where a bit is marginal, and goes bad after fabrication and testing are completed.
Therefore, it would be desirable to provide a structure and method for fabricating integrated circuits which provides for the detection of residual conductive material. It is also desirable that such a structure and method not significantly increase the complexity of the fabrication process.